Process for the preparation of diepoxides

ABSTRACT

Diepoxides of the general formula: ##STR1## wherein n has an average value not exceeding 0.03 and R is the bisphenyl radical of bisphenol-A are recovered from an epoxy resin of the same general formula with n varying from 0.07 to 0.30, by removing the low-boiling substance from the epoxy resin at 0.1-1 mm Hg and 165°-200° C and distilling off said diepoxide from the thus treated epoxy resin at 0.1-0.005 mm Hg and a temperature not exceeding 240° C. The low viscosity, narrow molecular weight distribution and excellent purity of these diepoxides allow their use in critical application fields.

The present invention relates to a process for recovering purediepoxides having low molecular weights and viscosities, from epoxyresins having medium or high values of the viscosity.

In the present specification, by epoxy resins having a medium or highviscosity, or merely by epoxy resins, will be intended the condensationproducts of bisphenol-A and epichlorohydrin, defined by means of thegeneral formula: ##STR2## wherein n has an average value of from 0.07 to0.30 and R is the radical of bisphenol-A (HO -- R -- OH).

Moreover, by diepoxides having low molecular weight and viscosityvalues, or merely by diepoxides, will be intended the products definedby means or formula (I), wherein n is zero or has an average value verynear to zero, up to a maximum of 0.03.

Epoxy resins are valuable products which find a number of applicationsin the art.

For example, they are used in the field of paints and coatings ingeneral, or else in the field of adhesives and binders (cement andbitumen pavements).

These resins are also used in the electronics field (casting, printedcircuits, sealing and encapsulation of electrical components) as well asin a number of other fields.

The production of epoxy resins from bisphenol-A and epichlorohydrin isknown in the art and may be carried out by means of a continuous ordiscontinuous process, in the presence of alkali metal hydroxide in anamount of 2 moles or about 2 moles, for each mole of bisphenol-A.

The discontinuous process is usually carried out by feeding aconcentrated aqueous solution of alkali metal hydroxide to a solution ofbisphenol-A in epichlorohydrin.

The reaction is carried out at atmospheric pressure, or at a pressureslightly lower than atmospheric, controlling the temperature in such amanner as to continuously distil off the water introduced together withthe alkali metal hydroxide, in the form of an azeotrope withepichlorohydrin.

Upon completion of the addition of the alkali metal hydroxide solution,all the residual water is removed, the unreacted epichlorohydrin isrecovered by distillation at subatmospheric pressure and the alkalimetal chloride, by-product of the reaction, is removed by dissolving itin water.

Epoxy resins are also prepared in the known art by means of a continuoustechnique, reacting bisphenol-A and epichlorohydrin in a plurality ofreactors arranged in series. More particularly, bisphenol-A andepichlorohydrin are continuously fed into the first of said reactors,whereas aqueous alkali metal hydroxide is fed into each of said reactorsup to a maximum amount equal to, or about equal to, 2 moles for eachmole of bisphenol-A.

The reaction products continuously discharged from the last reactor aredecanted to separate the liquid epoxy resin from the water and from thealkali metal chloride obtained as a by-product of the reaction.

A peculiarity of these known processes consists in that the reaction iscarried out in the presence of oxygen-containing organic substancesusually of the alcohol or ketone type.

As is known, in the synthesis of epoxy resins from bisphenol-A andepichlorohydrin, it is difficult to obtain products having a lowmolecular weight, corresponding to formula (I) with n equal to zero orat least near to zero.

In particular, the commercial epoxy resins obtained by reactingbisphenol-A and epichlorohydrin are usually liquid under ambientconditions and have an average value of n of from about 0.15 to about0.30.

These resins have moreover an epoxy equivalent value (grams of resinwhich contain one epoxy group) of from 190 to 210 and a viscosity at 25°C. of from about 10,000 to about 40,000 cps.

A typical molecular weight distribution of said epoxy resins is asfollows:

80-86% with a molecular weight of 340 (n= 0)

14-11% with a molecular weight of 624 (n= 1)

6-3% with a molecular weight of 908 (n= 2).

Attempts have been made in the art to reduce the value of n by means ofvarious expedients, such as for example by maintaining high values ofthe molar ratio between epichlorohydrin and bisphenol-A, or else, in thecontinuous processes, by dividing the alkali metal hydroxide amongst thedifferent reaction steps and by adding alcohols or ketones to thereaction medium.

These expedients have not given completely satisfactory results withregard to the molecular weight and the viscosity of the epoxy resins.

It has not been possible in practice to lower the epoxy equivalent belowabout 180 (n= 0.07 in formula (I)) and the viscosity below 7,000 cps(25° C.).

These epoxy resins moreover contain various impurities, especiallyunreacted monomers, monoepoxides, polyepichlorohydrins, in addition tothose deriving from the utilization in the synthesis, of organicsubstances different from the reagents proper.

A need was thus felt for obtaining epoxy resins endowed with a highpurity and having a molecular weight distribution as narrow as possible,in addition to a low viscosity.

A narrow distribution of the molecular weight and a low viscosity aredesirable since the compositions containing epoxy resins with thesecharacteristics are the best suited for the major part of theapplications, especially those in which one uses inert fillers.

On the other hand epoxy resins free, or substantially free, from theimpurities previously alluded to, permit the obtaining of manufacturedproducts having suitable characteristics even in the most criticalapplications, such as for example, in the electronics field.

It has now been found that it is possible to treat the epoxy resinshaving medium or high viscosities to separate a diepoxide having anextremely high purity, a low viscosity, and corresponding to astructural formula (I) is which n is zero or has an average value nearto zero, up to a maximum of 0.03.

Thus, the invention provides a process for recovering a diepoxide havinga viscosity of from 3,000 to 4,600 cps at 25° C., defined by means ofthe formula: ##STR3## wherein R is the bisphenyl radical of bisphenol-A(HO--R--OH) and n has an average value not exceeding 0.03, from a liquidepoxy resin having a viscosity of from 7,000 to 40,000 cps at 25° C.,defined by means of formula (I) wherein n has an average value of from0.07 to 0.30, characterized by removing in a first evaporation step thelow-boiling substances from said epoxy resin at a pressure of from 0.1to 1 mm Hg and at a temperature of from 165° to 200° C., and distillingoff said diepoxide from the thus treated epoxy resin, in a secondevaporation step, at a pressure of from 0.1 to 0.005 mmHg and at atemperature not exceeding 240° C., the overall residence time of saidepoxy resin in said first and second evaporation steps not exceedingabout 100 seconds.

The present invention will now be more fully described, by way ofexample only, with reference to the accompanying drawing, whichillustrates an apparatus for carrying out an embodiment of theinvention.

The epoxy resins usually treated according to the process of the presentinvention are commercial epoxy resins, corresponding to the generalformula (I) with n varying from about 0.15 to about 0.30 and having anepoxy equivalent of from 190 to 210 and a viscosity at 25° C. of fromabout 10,000 to about 40,000 cps.

According to the process of the invention, the epoxy resins are freed inthe first evaporation step from dissolved gases (more particularlyoxygen, carbon dioxide and nitrogen) and from low-boiling products (suchas epichlorohydrin and the organic substances used in the synthesisstage and the separation stage of the epoxy resins).

The first evaporation step is conveniently carried out in an apparatuspermitting a low residence time under the evaporation conditions to bemaintained, especially in thin falling film evaporators of the static ordynamic type.

In particular, operating under the conditions previously described, anamount of distillate of from 0.5 to 1.5% by weight with respect to theepoxy resin fed in is generally vaporized and the condensable fractionis conveniently recovered from the distillate by cooling at atemperature close to 0° C.

The second evaporation step is conveniently carried out in a guidedrotating film evaporator with internal condensation, which permitsextremely short residence times to be obtained under the evaporationconditions.

The operation temperature in said second step must not exceed 240° C.and it is generally not convenient to lower the temperature below 180°C.

The amount of recovered diepoxide obviously depends on the moleculardistribution of the epoxy resin under treatment and the value of n whichis desired in said diepoxide and generally ranges from 60 to 85% byweight with respect to the feed to the second evaporation step.

In any case, the evaporation times in the first and second steps, orbetter, the overall residence time of the epoxy resin under theevaporation conditions, are critical.

As indicated above, said residence time must be lower than 100 sec. andthe evaporation time is generally maintained at from 30 to 60 seconds inthe first step and from 20 to 40 seconds in the second step.

Thus, the process of the present invention comprises a first evaporationtreatment for the purpose of the removal from the epoxy resin of thegaseous products and the low-boiling products which are always presentin the epoxy resins, through in low amounts, as by products of thesynthesis or as residue of the solvents or diluents used in saidsynthesis or in the separation and washing steps of the resin.

This first evaporation step is essential for the purposes of the presentinvention to ensure in the subsequent evaporation step the vapourpressure conditions suitable for the evaporation of the diepoxide.

This latter evaporation should moreover be carried out in an apparatusallowing high heat-exchange coefficients, very short residence times andrelatively low evaporation temperatures under the operation pressure,thereby to permit a molecular distillation of the product.

In any case, operating under the described conditions, one primarilyavoids the polymerization phenomena which would lead to an increase inviscosity of the bottom products and accordingly to a loss in usefulproduct. The side reactions of aperture of the epoxy bridge, which wouldlead to a decrease in purity of the diepoxide and thus to a decrease ofthe technological value of said diepoxide, are also avoided.

Finally, operating according to the process of the present invention,one avoids the thermal scission reactions which would give rise toproducts of such structures as to adversely affect the properties of thecorresponding cross-linked epoxy compositions.

More particularly, the diepoxide obtained according to the process ofthe invention has typically a molecular weight equal to or very near to340, an epoxy equivalent of 170 or very close to this value and aviscosity at 25° C. of 3000-4600 cps, in addition to very high purity.

In practice, the average n value of the diepoxide is from 0 to 0.03 andusually from 0 to 0.01, and in this last case the viscosity is of theorder of 3000-3800 cps at 25° C.

In the following experimental examples, the apparatus schematized in thedrawing has been used.

More particularly, with reference to said drawing the epoxy resin is fedtoevaporator 10 through pipe 14, after pre-heating in exchanger 16.

Evaporator 10 is a commercial thin falling film evaporator of thedynamic type.

The distillate is removed through pipe 18, cooled in surface exchanger20 and finally recovered through pipe 22. Exchanger 20 is connectedthrough vacuum pipe 24 to ejector 28 in turn connected to exchanger 29.

The epoxy resin freed from low-boiling products is discharged throughpipe 30 and fed to evaporator 12 of the guided rotating film type,commercialized under the name ROTAFILM LH130 by the Carl-Canzler Society(Duren). Evaporator 12 is of the internal condensation type and thecondensed diepoxide is discharged through pipe 46. The high-boilingproducts obtained as the evaporation residue are recovered through pipe42after cooling in exchanger 44. Evaporator 12 is connected in series bymeans of pipe 32 to exchanger 34, pump 36, ejector 38 and exchanger 40.

EXAMPLE 1

There is used a liquid epoxy resin, obtained by condensation ofbisphenol-Awith epichlorohydrin, having the following characteristics:epoxy equivalent 192: viscosity at 25° C. 15,200 cps.

With reference to the accompanying drawing, evaporator 10 is maintainedat a pressure of 0.25 mm Hg and the temperature of the heating medium(Dowtherm) is maintained at a value such as to ensure a bottomtemperatureof 185° C.

Under these conditions 1.5% by weight of the feed is evaporated.

An epoxy resin having a viscosity at 25° C. of 15,350 cps and an epoxyequivalent of 190 (n= 0.15), is recovered at the bottom of evaporator 10through pipe 30. This resin is fed to evaporator 12 in whichthe pressureis maintained at 0.07-0.009 mm Hg and the exchange liquid in the jacketis maintained at a temperature such as to ensure a bottom temperature ofabout 230° C.

Under these conditions, the diepoxide condensed at the internalcondenser (maintained at about 50° C.) and collected in the distillateflask,is recovered through pipe 46 in an amount equal to 82% by weightwith respect to the feed to evaporator 12.

This diepoxide shows an epoxy equivalent value of 175 (n= 0.03) and aviscosity at 25° C. of 4,600 cps.

The residue having a viscosity at 25° C of 7.10⁵ cps and an epoxyequivalent of 265, is discharged at the bottom of the evaporator throughpipe 42.

EXAMPLE 2

The procedure is as in the first Example with regard to the epoxy resinused and the way of carrying out the first evaporation step. The resindischarged through pipe 30 is fed to evaporator 12 operated at 0.01 mmHg and at a bottom temperature of about 195° C.

Under these conditions a residue equal to 64.5% by weight with respectto the feed, having an epoxy equivalent of 200 and a viscosity at 25° C.of 20,500 cps, is discharged through pipe 42.

The diepoxide recovered through pipe 46 has an epoxy equivalent of 170(n= 0) and a viscosity at 25° C. of 3,800 cps.

EXAMPLE 3

One uses a liquid epoxy resin, obtained from bisphenol-A andepichlorohydrin, having an epoxy equivalent value of 185 and a viscosityat 25° C. of 9,000 cps.

Evaporator 10 is operated at a pressure of 0.5 mm Hg and with atemperatureof the heating medium such as to ensure a bottom temperatureof 170°C.

Under these conditions, an amount of 0.5% by weight with respect to thefeed is evaporated.

In a second run, evaporator 10 is maintained at a pressure of 0.5 mm Hgandat a bottom temperature of 190° C. In this case the amount ofdistillate is equal to 0.3% by weight with respect to the feed.

In a third run, evaporator 10 is maintained at a pressure of 0.3 mm Hgand at a bottom temperature of 200° C. The amount of distillate is equalto 1.3% by weight with respect to the feed.

EXAMPLE 4

The evaporation residue obtained in Example 3, third run, is an epoxyresinhaving an epoxy equivalent of 184 (n= 0.1) and a viscosity at 25°C.of 9,200 cps.

This epoxy resin is fed to evaporator 12 maintained at a pressure of0.09 mm Hg and at a bottom temperature of 200° C.

Under these conditions, an amount of diepoxide of 5.7% by weight withrespect to the feed is evaporated and this diepoxide has an epoxyequivalent value of 170 (n= 0) and a viscosity at 25° C. of 3,000 cps.The evaporation residue has an epoxy equivalent of 190 and a viscosityat 25° C. of 12,000 cps.

In a second run evaporator 12 is operated at 0.02 mm Hg and at a bottomtemperature of 220° C.

Under these conditions, an amount of diepoxide equal to 63% by weightwith respect to the feed is evaporated and this diepoxide has an epoxyequivalent value of 171 (n= 0.007) and a viscosity at 25° C of 3,800cps. The residue has an epoxy equivalent of 260 and a viscosity at 25°C. of 14,000 cps.

EXAMPLE 5

One uses a liquid epoxy resin obtained from bisphenol-A andepichlorohydrin, having an epoxy equivalent of 215 and a viscosity of25° C. of 40,600 cps.

This epoxy resin is fed to evaporator 10 which is operated at 0.2 mm Hgandat a bottom temperature of 195° C.

The amount of distillate is equal to 1.5% by weight with respect to thefeed.

The residual epoxy resin has an epoxy equivalent of 212 (n= 0.3) and aviscosity at 25° C. of 41,000 cps. This resin is fed to evaporator 12which is operated at a bottom temperature of 230° C and at a pressure of0.03 mm Hg.

The evaporated dioxide, equal to 68% by weight with respect to the feed,iscondensed at 50-55° C. This diepoxide has a viscosity at 25° C. of3,350 cps and an epoxy equivalent of 172 (n= 0.01).

The evporation residue has a viscosity at 25° C. of 8.10⁵ cps and anepoxy equivalent of about 315.

In all the examples, the residence time in evaporator 10 was of theorder of 30 seconds and that in evaporator 12 was also of the order of30 seconds.

We claim:
 1. A method for recovering a diepoxide having a viscosity offrom 3,000 to 4,600 cps at 25° C., defined by means of the formula:##STR4## wherein R is the bisphenyl radical of bisphenol-A (HO--R--OH)and n has an average value not exceeding 0.03, from a liquid epoxy resinhaving a viscosity of from 7,000 to 40,000 cps at 25° C. defined bymeans of formula (I) wherein n has an average value of from 0.07 to0.30, characterized by removing in a first evaporation step thelow-boiling substances from said epoxy resin at a pressure of from 0.1to 1 mm Hg and at a temperature of from 165° to 200° C., and distillingoff said diepoxide from the thus treated epoxy resin, in a secondevaporation step, at a pressure of from 0.1 to 0.005 mm Hg and at atemperature not exceeding 240° C., the overall residence time of saidepoxy resin in said first and second evaporation steps not exceedingabout 100 seconds.
 2. The method of claim 1, wherein said firstevaporation step is carried out in a thin falling film evaporation ofthe static or dynamic type.
 3. The method of claim 1, wherein theresidence time of said epoxy resin in said first evaporation step isfrom 30 to 60 seconds.
 4. The method of claim 1, wherein an amount oflow-boiling substances of from 0.5 to 1.5 wt.% with respect to saidepoxy resin is removed in said first evaporation step.
 5. The method ofclaim 1, wherein said second evaporation step is carried out at atemperature of from 180° to 240° C.
 6. The method of claim 1, whereinsaid second evaporation step is carried out in a guided rotating filmevaporator with internal condensation.
 7. The method of claim 1, whereinthe residence time of said treated epoxy resin in said secondevaporation step is from 20 to 40 seconds.
 8. The method of claim 1,wherein an amount of diepoxide of from 60 to 85 wt.% with respect tosaid treated epoxy resin is distilled off in said second evaporationstep.
 9. The method of claim 1, wherein said diepoxide distilled off insaid second evaporation step has an average value of n of from 0 to 0.01and a viscosity at 25° C. of from 3,000 to 3,800 cps.